Device for data exchange between a transmitter and a receiver

ABSTRACT

The inventive data exchange device comprises a transmitter (SA 4 ) fed by a power supply (VDDA), an electric cable (C 1 ) whose first conducting wire is connected to a fixed potential point (GNDA) of the transmitter and second conducting wire is connected to a variable potential point of the transmitter and a receiver (SB 4 ). Said receiver (SB 4 ) comprises a component (DZB 4 ) which defines a voltage threshold opposite to the direction of electric current in the cable (C 1 ). Said device is embodied in such a way that it is simple and low-cost in the production thereof. The device makes it possible to interconnect a plurality of transmitters and receivers and is low sensitive with respect to voltage and parasite currents.

FIELD OF THE INVENTION

The invention relates to an installation for exchanging informationcomprising a transmitter supplied from a power supply, an electric cableof which a first conductor is connected to a point of fixed potential ofthe transmitter and of which a second conductor is connected to a pointof variable potential of the transmitter and at least one receiver.

Such installations are widely used for exchanging information. Theyrequire, on the one hand, the use of shielded cables or pairs of twistedwires, protected against electromagnetic radiations and, on the otherhand, the use of circuits for generating signals constituting theinformation and for shaping these signals. Data transmissioninstallations using the EIB (registered trademark), LONWORKS (registeredtrademark) or RS485 standards, for example, are known. Such systems arevery competitive and make it possible to transmit information with ahigh bit rate. However, these installations are overdimensioned forcertain applications in which, in particular, a high bit rate is not animportant criterion.

PRIOR ART

Simpler installations are known from the prior art. For example, anassembly such as that represented in FIG. 1 is known. This assemblycomprises a transmitter SA1 and a receiver SB1 linked to one another byan electric cable C1 with two conductors whose electrical resistancesare symbolized by the resistors RL1 and RL2. The transmitter mainlycomprises a controlled switch consisting of a transistor TA1 operatingin switch mode and making it possible to connect together or not the twoends of the conductors of the electric cable. The receiver SB1 itselfcomprises a power supply providing a voltage VDDB linked to the end ofone of the conductors of the electric cable via a resistor RB1. Avoltage Us is measured between the ends of the conductors of theelectric cable. This voltage Us varies according to the state of thetransistor of the transmitter SA1. Thus, an item of information is codedas a succession of states of the transistor TA1 at the level of thetransmitter and decoded by measuring the variations of the voltage Us atthe level of the receiver SB1. When the transistor TA1 is on, thecurrent intensity in the electric cable linking the transmitter and thereceiver is mainly limited by the resistor RB1. The information bit ratebeing fairly low, it is unnecessary to represent on this diagram thecapacitive and inductive effects of such an arrangement.

Such an installation has drawbacks. Specifically, if one envisagesconnecting 100 receivers with the transmitter SA1 on the same line, thecurrent limiting resistors are arranged in parallel and their value thenequals RB1/100. In order to avoid causing overly large currents to flowthrough the transistor TA, it is then necessary to limit the number ofelements that can intercommunicate or to choose a large resistance RB1,for example 100 times the value causing the maximum current allowable bythe transistor TA.

It is known that in such installations, common-mode voltages anddifferential-mode voltages appear at the level of the transmitters andreceivers, in particular when the latter are distantly separated.

The common-mode voltages are represented by the arrows Ucm. On accountof the resistance RB1, a common-mode current necessarily causes amodification of the voltage Us.

The differential-mode voltages are caused by currents Idm flowing aroundthe loop formed by the two conductors of the electric cable between thetransmitter and the receiver. These currents passing through theresistors RL1 and RL2 likewise contribute to modifying the voltage Us.

To reduce the effects of the interference induced currents Idm, shieldedor twisted electric cables are used. In addition, conductors exhibitinga very small resistance are used and the allowable distance separatingthe various elements of the installation is restricted.

A compromise must be found regarding the value of the resistor RB1. Itsvalue must be high so as to allow communication between a maximum ofelements and to maintain, when the transistor TA1 is on, a voltage Usbelow the upper threshold of the low logic value of any logic circuitusing this voltage. Conversely, its value must be small so as to limitthe effects of the induced currents.

Installations such as that represented in FIG. 2 are also known. Thisassembly comprises a transmitter SA2 and a receiver SB2 linked to oneanother by an electric cable C1 with two conductors whose electricalresistances are symbolized by the resistors RL1 and RL2. The transmitterSA2 mainly comprises a power supply providing a voltage VDDA supplying aresistor RA and a transistor TA2 arranged in series. The transistor TA2is controlled by a circuit (not represented) and operates in switchmode. The receiver SB2 mainly exhibits a resistor RB2 between the endsof the two conductors of the electric cable. The voltage Us is taken atthe terminals of this resistor. Thus, an item of information is coded asa succession of states of the transistor TA2 of the transmitter anddecoded by measuring the variations of the voltage Us in the receiverSB2. When the transistor TA2 is off, the current intensity in theelectric cable linking the transmitter and the receiver is mainlylimited by the resistor RA.

In this installation, the value of the resistor RB2 must, likewise, belarge so as to allow the connection of a large number of elements, thevalue of RA being given. It must also be much greater than the values ofthe resistors RL1 and RL2. However, the value of RB2 must be as small aspossible so as to reduce the effects of the common-mode anddifferential-mode induced currents.

SUMMARY OF THE INVENTION

The aim of the invention is to produce an installation for transmittinginformation alleviating the drawbacks cited and improving theinstallations known from the prior art. In particular, the inventionproposes to produce a simple installation whose manufacturing costs arelow, making it possible to interconnect numerous transmitters andreceivers that are insensitive to interference currents and voltages.

The installation for exchanging information according to the inventionis characterized in that the receiver or the receivers comprise acomponent defining a threshold voltage opposing the flow of the electriccurrent through the cable. Thus, the interference voltages must begreater than this threshold voltage in order to bring about the flow ofa current around the cable and be interpreted as information.

The dependent claims 2 to 5 define various alternative embodiments ofthe installation according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The appended drawing represents, by way of examples, two embodiments ofan installation for exchanging information according to the invention.

FIGS. 1 and 2 represent transmitter-receiver assemblies known from theprior art, linked by electric cables allowing the exchange ofinformation.

FIG. 3 represents a first embodiment of an installation for exchanginginformation according to the invention, comprising a transmitter and areceiver linked by an electric cable.

FIG. 4 represents a second embodiment of an installation for exchanginginformation according to the invention, comprising a transmitter and areceiver linked by an electric cable.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The first embodiment of the installation for exchanging informationaccording to the invention is represented in FIG. 3 and comprises atransmitter SA3 and a receiver SB3 linked to one another by an electriccable with two conductors whose electric resistances are symbolized bythe resistors RL1 and RL2. The transmitter SA3 comprises a transistorTA3 and is identical to the transmitter SA2 previously described. Thereceiver SB3 comprises a voltage supply VDDB supplying a resistor RB4and a transistor TB3 arranged in series. The base of the emitter TB3 islinked to an end of a conductor of the electric cable via a DC voltagegenerator P3 such as a dry-cell or an electric accumulator and aresistor RB3. The transistor TA3 is controlled by a circuit (notrepresented) and operates in switch mode. The voltage Us is gatheredbetween the emitter and the collector of the transistor TB3.

Thus, an item of information is coded as a succession of states of thetransistor TA3 of the transmitter and decoded by measuring thevariations in the voltage Us in the receiver SB2. When the transistorTA3 is off, the current intensity in the electric cable linking thetransmitter and the receiver is mainly limited by the resistor RB3.

With such an arrangement it is possible to choose a large resistance RB3while being insensitive to the effects of the induced currents. Thetransistor TB3 remains, in fact, off when the differential-mode voltagedoes not become greater than the voltage of the generator P3 plus thevoltage between the base and the emitter of the transistor TB3.

For example, the voltages VDDA and VDDB of the power supplies of thetransmitter SA3 and of the receiver SB3 may be equal to 12 V. Theresistances RA and RB4 may be taken equal to 1 kΩ and RB3 equal to 50 kΩso as to allow the interconnection of numerous elements. The voltage ofthe generator may be taken equal to 4.5 V and the base-emitter voltageof the transistor TB3 equal to 0.6 V.

Thus, the differential voltage allowing the change of state of thetransistor equals substantially 5 V. This value gives a good safetymargin making it possible to prevent the effects of the inducedcurrents.

Such an arrangement has the drawback of using a DC voltage generatorsuch as an electric cell or an accumulator. In the latter case, it willbe noted however that the accumulator is recharged continuously acrossthe resistors RA, RL1, RB3 and RL2 when the transistor TA3 is open,thereby compensating for autodischarge and giving the component a longlifetime.

In such a circuit it is not possible to replace this generator with aZener diode of Zener voltage equal to 4.4 V in order to circumvent theinterference voltages.

Specifically, if the generator P3 is replaced with a Zener diode, whenthe transistor TA3 is off, the current is mainly limited by the resistorRB3 and equals substantially: (12−5)/50=0.140 mA. Such a low value hasthe consequence that the voltage across the terminals of the Zener diodeis very different from the Zener voltage and is in this casesubstantially zero. As a result, the circuit is sensitive to the inducedcurrents.

A second embodiment of an installation, represented in FIG. 4, makes itpossible to solve this problem. This installation comprises atransmitter SA4 and a receiver SB4 linked together by an electric cableC1 with two conductors whose electrical resistors are symbolized by theresistors RL1 and RL2. Information consisting of electric signals sentover the electric cable may be transmitted by the transmitter SA4 and bereceived by the receiver SB4. A single transmitter and a single receiverhave been represented in FIG. 4 with the aim of simplification andclarity. However, it is obvious that the installation can compriseseveral command transmitters and several command receivers linked inparallel on the electric cable. For example, in a home automationnetwork, such an installation allows communication between controldevices, electrical equipment and sensors. Each of its elements cancomprise a transmitter and a receiver so as to be able to carry outbidirectional communications between them.

The transmitter SA4 mainly comprises a power supply providing a voltageVDDA supplying a resistor RA and a transistor TA4 arranged in series.The transistor TA4 is controlled by a circuit (not represented) andoperates in switch mode. The receiver SB4 comprises a power supplyproviding a voltage VDDB and supplying a resistor RB6 and a Zener diodeDZB4 arranged in series with two parallel branches comprisingrespectively, a resistor RB7, and a transistor TB4 and a resistor RB8arranged in series. One of the two ends of the conductors of theelectric cable is connected between the resistor RB6 and the Zenerdiode, the other is connected to the base of the transistor TB4 by wayof a resistor RB5.

The installation may be embodied, for example, with the followingvalues:

VDDA=VDDB=12 V

RA=RB6=1 kΩ

UZ=3.9 V

RB5−47 kΩ

RB7=4.7 kΩ

RB8=100 kΩ

An item of information which is to be sent from the transmitter SA4 tothe receiver SB4 is coded as a temporal succession of off and on statesof the transistor TA4. It is decoded in the receiver SB4 by analyzingthe variations in the voltage Us that is measured across the terminalsof the resistor RB8.

When the transistor TA4 is off, a voltage is present between thecollector and the emitter of the transistor TA4. This voltage equalssubstantially some 12 volts. It causes the flow of a current passingthrough the resistor RL1, the Zener diode DZB4, the transistor TB4, theresistor RB5 and the resistor RL2. The Zener voltage of the diode DZB4is maintained by a current flowing across the resistors RB6 and RB7wired between the power supply terminals of the receiver SB4. This Zenervoltage and the emitter-base voltage of the transistor TB4 oppose theflow of the current through the cable. When a sufficient current flowsthrough this cable, the transistor TB4 is on and the voltage Us thenequals some 10 volts and is interpreted as a high state by a logiccircuit. The large value of the resistor RB5 allows limitation of thecurrent and the possibility of connecting a transmitter with numerousreceivers.

When the transistor TA4 is on, the voltage between its collector and itsemitter is substantially zero. This has the consequence that no currentflows around the loop. The transistor TB4 is consequently off and thevoltage Us is substantially zero. This is interpreted as a low state bya logic circuit. The current passing through the Zener diode and makingit possible to maintain the Zener voltage at its terminals is chosen tobe around 10 times greater than the induced currents that may beencountered in the cable. This makes it possible to ensure that inducedinterference currents cannot make the transistor TB4 switch into an onstate.

Such an installation comprising some 100 receivers and a length ofconnection cable of 1000 m operates perfectly.

The transmitters and the receivers may of course comprise other elementssuch as capacitors. The transistor TA4 may, for example, be controlledby a microcontroller.

To allow the connection of an even larger number of elements in theinstallation, the resistor RA can also be replaced by a transistor.

1. An installation for exchanging information comprising a transmitter(SA3; SA4) supplied from a power supply (VDDA), an electric cable (C1)of which a first conductor is connected to a point of fixed potential(GNDA) of the transmitter and of which a second conductor is connectedto a point of variable potential of the transmitter and at least onereceiver (SB3; SB4), wherein the receiver or the receivers (SB3; SB4)comprise a component (P3; DZB4) defining a threshold voltage opposingthe flow of the electric current through the cable (C1).
 2. Theinstallation for exchanging information as claimed in claim 1, whereinthe component (P3) defining a threshold voltage opposing the flow of theelectric current through the cable (C1) is a dry-cell or an electricaccumulator (P3).
 3. The installation for exchanging information asclaimed in claim 1, wherein the component (DZB4) defining a thresholdvoltage opposing the flow of the electric current through the cable (C1)comprises a Zener diode (DZB4) supplied with a continuous current, suchthat between its terminals it exhibits a voltage substantially equal toits Zener voltage even in the absence of current in the cable (C1). 4.The installation for exchanging information as claimed in claim 3,wherein the threshold voltage opposing the flow of the electric currentthrough the cable (C1) is the sum of the Zener voltage of the Zenerdiode (DZB4) and of the emitter-base voltage of a transistor (TB4) whoseemitter is linked to the anode of the Zener diode (DZB4).
 5. Theinstallation for exchanging information as claimed in claim 1, whereinthe threshold voltage is greater than 2 volts.
 6. The installation forexchanging information as claimed in claim 2, wherein the thresholdvoltage is greater than 2 volts.
 7. The installation for exchanginginformation as claimed in claim 3, wherein the threshold voltage isgreater than 2 volts.
 8. The installation for exchanging information asclaimed in claim 4, wherein the threshold voltage is greater than 2volts.